The US Boston Metro is the first time that China’s track equipment has landed in the United States,and also the first time that China’s equipment industry has realized technology export to the world’s high-end market. This project is the first time that laser overlay welding technology has been formally applied to railway cars in china. This paper combines the structural characteristics of laser overlay welding of stainless steel body,develops EMAT automatic detection technology on-line inspect the quality of laser overlay welding of stainless steel car body,which is used to inspect and evaluate the fusion depth of laser overlay welding of stainless steel car body. The application of EMAT technology in laser overlay welding of stainless steel car body is studied. It has the characteristics of no coupling agent,high detection accuracy and difficult wear of probe. It can realize automatic detection and evaluation for laser overlay welding of stainless steel,avoid the problems of poor welding. It is of great significance to ensure the quality of laser overlay welding of stainless steel.